The disclosure relates generally to turbine technology. More particularly, the disclosure relates to a variable area nozzle, for use in a multi-stage turbine.
In the design of gas turbine engines, fluid flow through the engine is varied by a plurality of stator vanes and rotor blades. Typically, static nozzle segments direct flow of a working fluid into stages of turbine blades connected to a rotating rotor. Each nozzle has an airfoil or vane shape configured such that when a set of nozzles are positioned about a rotor of the turbine, they direct the gas flow in an optimal direction and with an optimal pressure against the rotor blades.
Directional and pressure requirements may vary with changes in operating conditions including temperature, engine mass flow, and so forth. Static vanes may not provide optimal direction and pressure over a full range of operating conditions, resulting in decreased efficiency and/or a harsher than necessary environment for components. Further, static vanes have a finite lifespan, due to the harsh environment inside a turbine, which may be maintained at significant pressure and temperature, e.g., 982-1093° C. (1800-2000° F.). Repair and replacement of static vanes typically requires disassembly of a turbine, which is costly in both labor and down time for the machine.
A number of designs have incorporated variable vanes in an effort to enhance flow direction and pressure. Variable vanes have been used having a hollow passage configured to accommodate a support strut and an inner strut, and to provide cooling air flow to the inner strut in the vicinity of the variable vane. Rotation of the vane to adjust angle has been accomplished through sleeve bearings. However, this design may fail to address prolonged field operation due to wear issues on mating components, and may require regular overhaul.
Other designs have been used, including a variable area turbine entrance nozzle having moveable vanes which are rotated in the middle stage of a turbine engine. The moveable vanes are sealed against the outer casing and the rotor to prevent leakage of air therethrough. This design may also be unsuitable for prolonged field operation, however, and regular overhauls are costly in both labor and turbine down time.